Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. After purification and beneficiation, kaolin clay is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
Crude kaolin clay, as mined, contains various forms of discoloring impurities, two major impurities being anatase (TiO.sub.2) and iron oxides. To make the clay more acceptable for use in the paper industry, these impurities must be substantially removed by appropriate techniques.
The production of high brightness clays usually includes at least two processing steps. In a first step, a significant portion of the impurities, mainly anatase, is removed by employing one or more physical separation techniques, such as centrifugation, sedimentation, high gradient magnetic separation, froth flotation and/or selective flocculation. The remaining impurities, mainly iron oxides, are subsequently removed by known techniques, such as chemical leaching.
Froth flotation is regarded as one of the most efficient methods for removing colored impurities from kaolin clay. Typically, clays to be beneficiated by froth flotation are first blunged in the presence of a dispersant and pH modifier and then conditioned with a collector. The job of the collector is to selectively adsorb to impurities and render them hydrophobic. This part of the process is referred to as conditioning. The conditioned impurities, mainly titanium dioxide in the form of iron-rich anatase, are then removed in a flotation machine via the attachment of the hydrophobic impurities to air bubbles which are injected into the feed slurry or into the flotation pulp.
Two general categories of compounds are reported in the literature as collectors for titaniferous impurities in kaolin clay. Cundy U.S. Pat. No. 3,450,257 discloses the use of fatty acid compounds as collectors, and Yoon & Hilderbrand U.S. Pat. No. 4,629,556 discloses the use of hydroxamate compounds as collectors. Each category of compounds has advantages and disadvantages.
However, conventional flotation processing is adversely affected by the dispersant and pH modifier which tend to disrupt the mechanism by which the collector attaches to the impurities. This and other problems encountered when dispersing (i.e., deflocculating a crude kaolin clay for flotation beneficiation) are discussed in Hunter U.S. Pat. No. 3,410,399.
Therefore, a need exists in the kaolin clay industry for a process which will effectively condition kaolin clays for subsequent removal of impurities, such as in a flotation process.